Effective charge of high-generation PAMAM dendrimers in the adsorbed state

The adsorption of macromolecules onto charged surfaces is an ubiquitous process in nature and industrial processes; it has, for example, important implications for surface modification and sensors in analytics. In the case of proteins and some polyelectrolytes, such as dendritic ones, the adsorption process and the resulting final coverage can be described within the framework of the random sequential adsorption (RSA) model. To provide a quantitative description of the intermolecular interaction potential, the effective charge of the molecules is of great importance. The incorporation of charge effects results in the so-called extended or electrostatic RSA model. Here, it is demonstrated for poly(amidoamine) (PAMAM) dendrimers of generation 10 that the influence of the substrate on the effective charge must be considered. Streaming potential measurements of PAMAM G10 adsorbed onto mica and silica showed that the effective charge differed significantly for both substrates. Moreover, for substrates whose surface charge varies with pH, the effective charge varies not only by the titration of the ionizable groups of the polyelectrolytes but also due to the electrostatic interaction with the substrate. A first estimation to account for these effects is provided.